1. Field of the Invention
This invention relates to a toner used for developing an electrostatic latent image in electrophotography or electrostatic recording, a method for producing such a toner, a developer for developing an electrostatic latent image and a method for forming an image by employing such a developer.
2. Description of the Related Art
The dry developers used in electrophotography, etc. are classified into two main types: a one-component developer composed solely of a toner produced by dispersing a coloring agent in a binder resin, and a two-component one composed of a toner and a carrier. It is important for the developing agent to be excellent in flowability, transportability, fixability, chargeability and transferability in order to be suitable for use in a developing process. The toner particles contained in the developer are usually produced by a mixing and pulverizing process. This process, however, gives only a toner having an irregular particle shape and an undesirably broad particle size distribution.
A process including emulsion polymerization and cohesion has been proposed for producing a toner having a controlled particle shape and a controlled particle size distribution (Japanese Patent Applications Laid-Open Nos. 63-282752 (1988) and 6-250439 (1994)). According to this process, dispersion of resin particles is produced by emulsion polymerization, while dispersion of a coloring agent is produced by dispersing it in a solvent, these dispersion are mixed to form cohering particles having a diameter corresponding to that of toner particles, and the cohering particles are melted and united by heating to form toner particles. This process can advantageously form toner particles having any desired shape from irregular to spherical if an appropriately selected heating temperature is employed.
In connection with the above process including emulsion polymerization, we, the inventors of this invention, have proposed the addition of dispersion of resin particles as divided in a plurality of portions so that resin particles may adhere to the surfaces of cohering particles containing a mold release agent, etc. to form outermost resin layers to thereby prevent the mold release agent from being exposed on the surfaces of toner particles and make a toner having an improved powder property (Japanese Patent Application Laid-Open No. 10-26842 (1998)). It has, however, been impossible to incorporate all of the coloring agent in its dispersion into cohering particles and avoid the presence of any free coloring agent remaining in its dispersion. The free coloring agent adheres to the toner surfaces, and if carbon black is used as the coloring agent, the toner has a higher electrical conductivity in those parts thereof to which carbon black adheres, and the chargeability of the toner varies with the environment and its variation brings about undesirable results including a defective transfer.